In semiconductor processing, many operations may be performed on a single workpiece or semiconductor wafer. In general, each processing operation on a workpiece is typically performed in a particular order, wherein each operation waits until completion of a preceding operation. In many processing operations, a particular orientation of the workpiece and/or knowledge of the position of the workpiece with respect to a workpiece holder is needed in order to properly process or handle the workpiece. For example, operations such as an exchange of workpieces between transport carriers or storage cassettes and the processing system and a transfer of the workpieces from an atmospheric environment into an evacuated environment of a process chamber of the processing system through one or more load lock chambers may require specific orientation(s) or knowledge of the spatial position of the workpiece for proper workpiece handling and processing.
An orientation of the workpiece (e.g., notch alignment) may be performed within the evacuated environment or atmospheric environment via a light presence sensor, whereby a beam of light is emitted by a light emitter and directed toward the workpiece concurrent to a rotation of the workpiece with respect to the beam of light. A variation in light received by a light receiver can be then used to determine the position of notch defined in the workpiece and/or an eccentricity of a position of the workpiece, depending on how the light is fully or partially received. One such system is disclosed in U.S. Pat. No. 5,740,034 to Hiroaki Saeki, whereby a waveform associated with the received light signals is utilized to determine the position of the notch and/or eccentric position of the workpiece.
Conventionally, such positioning via light presence sensors has been adequate for accurately determining the position of a workpiece that is opaque to the light emitted, such seen in conventional silicon substrates. However, when substrates or workpieces undergoing processing in the same processing system differ materially from one another (e.g., silicon versus silicon carbide), the use of a conventional light presence sensor and aligner can lead to various errors in positioning, specifically when the substrate is partially transparent to the light emitted. For example, a difference in transmission from one substrate to another can lead a significant error in positioning using conventional alignment systems and methods. The transmissivity and emissivity from workpiece to workpiece can vary with the constituency, thickness and coatings disposed on the particular workpiece. As such, when changing from one workpiece type to another (or one coating to another), conventional alignment systems fail to provide adequate positioning data without significant changes to the alignment systems, themselves, such as changing the wavelength of light emitted therefrom.